As is generally well-known, gas turbine engines are continually undergoing changes with goals of improving performance, decreasing size and weight for a given thrust rating, while reducing cost and enhancing durability, producibility and repairability. To improve performance, it is typical to increase the operation temperature of the engine since increased turbine temperatures is tantamount to improved engine performance. Of course, these increased temperatures necessitate other changes in the engine to handle the increased thermals so as to maintain structural integrity of the engine's components.
In heretofore known engines, the turbine exhaust case and its struts that are disposed in the gas path are typically unshielded from the environment and hence, its durability would be predicated on the material selected. Some attempts have been made to encapsulate the struts with an aerodynamic fairing that would also serve as a heat shield for the strut by welding the fairing in situ. Obviously, since the welding is performed manually, one would expect to find variations from assembly to assembly and quality would be highly dependant on the skill of the welder. And, of course, the weldment, if not done properly, is susceptible to malfunction. As would be appreciated by one skilled in the art, the welded fairing presents a difficult fabrication, requiring much fit up at assembly and results in undesirable weld distortion with potential stress risers at the weld joints. Also, repair of locally cracked, eroded or otherwise damaged areas require "patch" welding or complete refabrication. This type of construction and consequent repair is therefore difficult and costly. Furthermore, the strut fairing is essentially a "one piece" welded construction and is life limited by inherent thermal stresses due to both radial and axial temperature gradients.
Further, with the use of a welded fairing the leading edge or "nose" which first sees the gas path flow stream is subjected to the major abuse and hence susceptible to a limited life. The hot gases in the gas path impinge on the nose, the nose is subjected to a high temperature profile since the hotter gas temperatures are toward the middle of the hot gas flow path and additionally it incurs potential hot streaks owing to circumferential unevenness of the temperature profile in the gas flow path.
Since the struts are designed to allow passage of the support rods and service lines that feed lubricating oil to the bearings, it is abundantly important that the heat shield protect those components of the engines. To this end, the lubrication lines must be sufficiently cool to prevent oil from coking. The support rods, likewise, must be sufficiently cool to prevent loss of preload which is set at assembly caused by temperature differences between the inner bearing compartment, the rods and the outer duct support/mount ring that may occur during engine operations.
We have found that we can obviate the problem noted above by providing a replaceable multi-piece fairing to the exhaust case. It is contemplated that the strut fairing will be fabricated in three parts wherein the nose is made from a separate single piece that is capable of being removed from the remaining portion of the assembly. Since the nose is first to see the hottest portion of the gas path and is susceptible to foreign object damage, it can be made of materials that resists these hazards which may be different from the material used in the other pieces.
Likewise, the material of the fairing can be different from the material of the structural parts of the turbine exhaust case. In this manner, the fairing can be made from materials that exhibit good thermal characteristics at elevated temperatures while the structural frame can be made from materials that exhibit higher strength with lower temperature capabilities.
Since the fairing is made from a number of pieces, these pieces can be readily removed so as to enhance the repairability of the assembly. Nor would the entire casing have to be removed to repair the fairing assembly.
It is further contemplated that the fairing not only serves to keep the structure at lower temperatures, it is radially free floating relative to the structural frame so as to minimize, if not eliminate, thermal stresses that would otherwise occur.